1. Field of the Invention
This invention is concerned with piezoelectric transducers for use in borehole logging, seismic exploration and similar research.
2. Discussion of Related Art
Piezoelectric transducers are well known for use in seismic exploration and borehole logging. The transducers may be designed to act as either a transmitter or a receiver or, perhaps, the same unit may be used for both purposes in alternate cycles. Typically, piezoelectric transducers employ ceramic materials such as barium titanate or lead zirconate titanate. The units may be polarized to operate in the bender mode or in the radial mode depending upon the desired use. The sensitive element may be a thin wafer backed by a metal diaphragm, a right cylinder or a flat slab mounted on an inert plate that is supported from one or both ends. Ceramic material is brittle and, in the presence of a shock wave as from a nearby air gun, a ceramic element may shatter.
Another class of piezoelectric material is polyvinylidine flouride (PVDF) which is a polymer film that is known by the trade name of KYNAR made by Atochem, a Division of Elf Aquataine, of Valley Forge, Pa. This film is preferred by some workers in the art because its acoustic impedance is close to that of water so that acoustic wavefields do not suffer spurious reflections from that material as they do when encountering a ceramic element. The signal strength is much higher than can be obtained from ceramic devices of similar physical size. The film is flexible and very strong so that a PVDF transducer is more rugged than a ceramic transducer.
Prior to use, the film must be poled in the thickness direction by soaking it in an electric field such as 20 volts/mil at an elevated temperature such as 200.degree. for a period of time such as 2000 seconds. The opposite sides of the film are metallized by evaporating thereon a thin conductive coating, a few microns thick, of metallic silver or silver ink. Other metals such as gold may also be used. Electrodes are bonded to the metallized backing on the opposite sides of the piezoelectric film for electrical communication with the outside world. An external force applied to the film results in a tensile strain or a compressive stress. In response to the stress/strain, the film develops a proportional open circuit voltage, that is, an electrical charge in proportion to changes in the applied mechanical force. The charge leaks away as a function of time depending upon the dielectric constant of the film and the impedance of the connected circuitry. By convention, the polarization axis is in the thickness direction. Tensile forces may be applied along either the longitudinal or the transverse axis.
U.S. Pat. No. 4,653,036, issued Mar. 24, 1987 to G. R. Harris et al teaches a hydrophone device that includes a piezoelectrically active sheet stretched and clamped over the top of a hoop ring. A backing is attached to the back of the hoop ring. A low dielectric material fills the space between the backing and the sheet. This material eliminates the capacitative loading effect which would otherwise be presented by the medium being probed.
Another acoustic wave sensor is described in U.S. Pat. No. 4,756,192, issued Jul. 12, 1988, to G. Heine et al. The shock wave sensor includes a piezoelectric foil that is either suspended in a ring or on a backing. Electrodes run from a particular sensitive area on the foil towards the edges and are fracture-proof connected to concentric conductors.
C. Bauducel et al, in U.S. Pat. No. 4,810,913, which issued Mar. 7, 1989, discloses a hydrophone that is provided having at least one flexible piezoelectric sensitive element applied to a support. Each sensitive element is formed of a flexible piezoelectric film associated with two electrodes and is applied to a synthetic plastic material support one face at least of which has hollow parts of very different shapes, for example a homogeneous distribution of cups, honeycombs, holes etc. whose dimensions are determined as a function of the mechanical characteristics of the films used. The stresses applied thereto result in deforming each film inwardly of the hollow parts which very greatly increases the sensitivity.
In this disclosure, the term "hydrostatic pressure" refers to the static pressure due to a fluid column of some particular height. The term "hydrodynamic pressure" refers to the cyclic pressure differences that are created by acoustic waves propagating through a fluid.
This invention in intended to fill a need for a rugged acoustic transducer that is simple in design and economical to manufacture.